The invention relates generally to a thermal mass flow controller and in particular relates to a thermal mass flow controller having a portion of a sensing element oriented transversely with respect to a bypass flow path extending through the thermal mass flow controller.
Thermal mass flow controllers and thermal mass flow meters are employed in the semiconductor industry and in other industries for measuring the rate of flow of a quantity of gas employed in a piece of equipment for the manufacture of a semiconductor wafer and the like. Such thermal mass flow controllers often are used in gas shelves of diffusion furnaces, chemical vapor deposition systems, plasma etching systems, sputtering systems and the like to meter precisely amounts of reactant and carrier gases to a working chamber of the equipment. The thermal mass flow controllers are used to meter precisely the amounts of reactant and carrier gases to be delivered to a treatment chamber of the equipment. Such treatment chambers may comprise process tubes or process chambers. Such gases may include hydrogen, oxygen, nitrogen, argon, silane, dichlorosilane, ammonia, phosphorus oxychloride, diborane, boron tribromide, arsine, phosphine, sulfur hexafluoride and the like. Oftentimes, multiple gas sources are employed in conjunction with a particular treatment chamber. For instance, silane may be used in the treatment chamber for chemical vapor deposition of polycrystalline silicon, also known as polysilicon, in combination with one or more doping agents. As a result, each of the process tubes or process chambers in a particular piece of equipment may have multiple reactant gas delivery lines connected and must, of necessity, have multiple mass flow controllers connected in the gas lines to meter appropriate amounts of the reactant and carrier gases process gases to the treatment chamber. The use of such multiple mass flow controllers, of course, expands the size of the gas shelves used for these types of equipment.
The manufacture of modern semiconductors having finer and finer microelectronic features has necessitated that the acceptable contamination levels within clean rooms in which such manufacturing takes place have continuously been reduced in order to provide adequate wafer yields. As a result, the expense involved in the construction of such clean rooms has steadily increased and is anticipated to continue increasing. As such clean rooms are expanded in size due to the relative amount of floor space or foot-prints occupied by equipment, their corresponding cost of course also increases. Thus, the equipment size for a given throughput through a particular clean room is an economic consideration which is always of importance to a wafer fabricator.
Concomitant with the space requirements for clean rooms is a requirement that footprint considerations often require that mass flow controllers be capable of use in a variety of orientations. Unfortunately, in most cases, conventional thermal mass flow controllers may only be used with their bypass and sensors both positioned substantially horizontally to avoid introducing unwanted convective effects into the sensor which would result in perturbation of the mass flow controller readings.
One approach to solving the convection problem is to allow a flow controller for instance to be oriented vertically, as set forth in PCT application PCT/US91/04208, published Dec. 26, 1991, corresponding to U.S. application Ser. No. 07/537,571, filed Jun. 14, 1990 now abandoned and corresponding U.S. application Ser. No. 07/614,093, filed Nov. 14, 1990 now abandoned all for Thermal Mass Flow Meter, assigned to the instant assignee. Those applications disclose a thermal mass flow meter having a sensor which allows the bypass flow path to be oriented in a substantially vertical direction without the necessity of the sensor being oriented in a substantially horizontal direction. The mass flow controller, however, like other prior art mass flow controllers may only be used in a vertically oriented direction. That is, it has a single preferred direction in which it may be oriented. It may not be used in a variety of attitudes other than with the bypass position substantially vertically.
U.S. Pat. No. 4,776,213 to Blechinger et al. discloses a mass airflow meter in a bypass which is transverse to the main air flow path.
What is needed then is a thermal mass flow controller which is compact and may be positioned in a variety of orientations with introducing convective perturbations in the flow controller reading.